Speed through water

[MexicanGulf]

Hi Guys,

There's something that's been on my mind for quite a while, and I think we don't talk about it enough: the actual speed of the lure through the water — the real speed, not the number we read on the GPS.
We usually focus on the boat speed — 2.5 knots, 3 knots, 6 knots, and so on — but that's only the speed over ground. In reality, the lure is moving through a fluid that's never still. There's current, wind drift, the boat's wake, the line angle... and all of this completely changes the picture.
I've been a longtime reader of Peter Pakula's work, a true master in my opinion. He's written about this subject several times, and his insights really sparked my interest in it.
More than once, both when slow trolling live bait and when pulling Kona-style lures, I've realized how little it takes to change the lure's behavior completely. Sometimes you slow down by half a knot and the lure comes alive; other times it just dies and stops swimming properly.
This is especially true with live bait, since I often practice slow trolling with live baits of different kinds. Some handle higher speeds pretty well — like large mackerel or small tunas — while others are far more delicate, like sardines, squid, or cuttlefish.
With those fragile baits, I've noticed how crucial it is to pay attention to trolling speed and direction in relation to the current.
That's what really gets me thinking and leaves me full of doubts and questions.
How can we truly know the actual speed of the bait through the water?
I don't mean the boat's GPS speed — I mean the bait's real movement relative to the mass of water around it.
There are countless variables: surface and subsurface currents, line angle, terminal weight, trolling direction versus current... everything plays a part.
Has anyone ever tried to measure or estimate this difference in a practical way? Maybe using flow sensors, underwater cameras, or just by carefully observing lure or bait behavior in different situations?
Trolling a live sardine at 150 feet deep, at one knot, with or against the current, completely changes how that bait travels and behaves through the water.
It would be great to gather some real-world experiences, because I honestly believe that understanding this aspect better can be the difference between a lure that just "runs" and one that actually catches fish.

 

[oc1]

how about this?

[MexicanGulf]

I had never heard of this thing

[Keta]

My chartplotter has a paddle wheel transducer for water speed and a GPS for land speed.

[oldmanjoe]

  Now they have video cameras that go on your line . You can see what your lure is doing .  How it swims action wise .  There was a video of a fish following the bait for a half mile before it struck the bait.
 

[MexicanGulf]

SOG vs STW: how to really read your speed and dial in your trolling game
Speed Over Ground (SOG) is the speed your boat's doing over the earth's surface — basically what your GPS shows. Speed Through Water (STW) is how fast you're actually moving through the water itself, picked up by paddle wheels or ultrasonic sensors like the Airmar DX900. Sometimes your STW can be zero while your GPS still shows 5 knots — that just means you're drifting with a 5-knot current. Usually, STW's what matters for radar collision avoidance since it gives a truer picture of your movement, while SOG's what you use for navigation.
Current direction and strength are huge factors for tons of fishing setups. The right trolling speed can totally make or break how your bait runs, but keeping that speed locked in can be tricky. Your trolling speed is the real in-the-water speed — that's your STW. The GPS just shows how fast you're moving over the ground, current included.
If you're running only a GPS to watch your speed, an easy trick is to drive a few circles and watch your readout. The graphic example makes it clear: going with the current you'll see your top speed, say 8.5 knots, and heading against it you'll see your slowest, maybe 4.5 knots.
Here's how you figure it out. Take the difference between the two numbers: 8.5 minus 4.5 equals 4 knots. Split that in half — that's 2 knots, your current speed. Now back-calculate your true trolling speed (STW): going with the current, 8.5 minus 2 gives you 6.5 knots; against it, 4.5 plus 2 gives the same 6.5 knots. So your actual trolling speed through the water is 6.5 knots either way.
Bottom line: SOG is what your GPS shows — speed over the ground, current included. STW is your real through-the-water speed, and that's the one that counts for keeping your trolling steady and your lures running right. Even if you're rolling with just a GPS, you can still estimate the current by running a circle, checking your fastest and slowest speeds, doing the math, and locking in your true STW.